This is the scan of a normal cat. Both lobes of the thyroid gland are visible.
Hyperthyroidism is a disease of older cats that we have diagnosed with increased frequency in recent years. It is almost always caused by a benign tumor of the thyroid gland that increases the amount of thyroxine (the hormone secreted by the thyroid gland) in the bloodstream. This increase in thyroxine causes an increase in the overall metabolism of the body, leading to problems for several internal organs. Even though this disease can be diagnosed in young cats, most cats that get this problem are older. Cats 8 years of age or older should be screened for this problem when routine blood panels are run.
Cats that have Feline Hyperthyroidism commonly have other problems that need careful attention if the thyroid problem is to be treated successfully. The excess thyroxine can cause these other problems, or make them worse if they already exist. Some of these other common problems are kidney disease, heart disease, dental disease, sugar diabetes (diabetes mellitus) and high blood pressure (hypertension) leading to blindness.
The symptoms that occur depend on which internal system or systems are most influenced by the increase in thyroxine circulating throughout the bloodstream. The more common ones are:
During a physical exam some cats will appear thin and have racing heart rates and even heart murmurs (click here if you would like to hear what a murmur sounds like). In some cats we can feel an enlarged thyroid gland. You might notice our doctors palpating your cat's neck and throat during an exam to check for this enlargement. Some cats can have an enlarged thyroid gland that can not be palpated because it has slipped from its normal position at the throat and repositioned itself further down the chest.
An important tool in the diagnosis of Feline Hyperthyroidism is a blood panel. We can sometimes detect the effects of the excess thyroid hormone on the internal organs by running a routine blood panel. In addition to this blood panel we run a specific thyroid test which is frequently all we need to make the diagnosis. Since thyroxine levels fluctuate, we occasionally need more sophisticated tests to make an accurate diagnosis.
One of the most precise tests to diagnose hyperthyroidism is a scan of the thyroid gland. Not only is this test accurate in diagnosing the problem, it will let us know if some of the abnormal thyroid tissue has repositioned itself further down the chest. An additional benefit is its ability to detect a malignant cancer of the gland in the rare occasion that this occurs.
This is the scan of a normal cat. Both lobes of the thyroid gland are visible.
This cat has Feline Hyperthyroidism. It is the more common benign version, and is present in only one lobe of the gland.
Currently there are three ways to treat hyperthyroidism: medical, with the use of anti-thyroid drugs; surgical removal of the tumor; and radiation, with the use of radioactive iodine (I 131). Regardless of which route of therapy is chosen, cardiac function should be stabilized in those hyperthyroid cats with severe tachycardia (rapid heart rate), cardiac arrythmias or pleural effusion (fluid in the chest). Chest x-rays may be taken to evaluate the heart size and to check if any fluid is present in the chest. Cardiomyopathy may be present secondary to the elevated levels of circulating thyroid hormone. An EKG maybe be taken to evaluate if arrythmias are present. Propanolol, a beta-blocker, can be used to control the cardiac signs. Once the thyroid state is successfully treated the cardiovascular abnormalities usually resolve and propanolol therapy can be discontinued.
Anti-thyroid drugs
Currently there are two anti-thyroid drugs available for the medical management of feline hyperthyrodism. Propylthiouracil (Propycil) and Methimazole (Tapazole). Both of these drugs inhibit the synthesis of thyroid hormone. Both are inexpensive, available, and usually effective in controlling hyperthyroidism. These drugs are used on cats with chronic kidney failure, a common problem as cats age. However, there are some disadvantages. Daily medication, usually twice a day, is necessary for the cat's life. Some cats are difficult to pill, and many owners don't like the idea of having to pill their cat.
These drugs merely correct the elevated thyroid hormone; they have no anti-tumor actions so the tumor may continue to enlarge. In a small number of cases these drugs become ineffective with time, resulting in recurrence of hyperthyroid signs. They are also capable of inducing adverse reactions including anorexia (appetite loss), vomiting, depression, lethargy, liver failure and bone marrow problems causing anemia and reduced platelet numbers. Bone marrow problems usually occur 2-8 weeks after anti-thyroid therapy is initiated, therefore CBC and platelet counts are recommended every 2 weeks for the initial 2 months of therapy. Fortunately, these adverse reactions are less common with Tapazole than Propycil, so Tapazole is considered the drug of choice. Cost is approximately $250-$450 per year for the drugs.
Surgery
Surgery is also used to treat this condition, and can be very effective. It has the advantage of a rapid and successful cure rate. Anti-thyroid therapy may be used as an initial treatment to stabilize the hyperthyroid cat prior to surgery. After 2-4 weeks of therapy, many of the metabolic and cardiovascular derangements will resolve, making the aged cat a more acceptable anesthetic and surgical risk. The thyroid tumor may then be removed and the anti-thyroid medication discontinued. The advantage of surgery is the relatively easy procedure involved and the potential for a cure. The disadvantages of surgery include the risks of anesthesia in an old cat and the potential development of hypoparathyroidism or recurrent laryngeal nerve damage following surgery. Complications are not common however. Costs for surgery is $500-$700.
Radioactive Iodine Therapy
Failure to correct the hyperthyroid state following surgery is an indication that either the other thyroid gland is tumorous or there is tumor tissue present in the lungs or anterior mediastinum (the tissue just in front of the heart). About 5% of cats will have a tumor in the chest. In these cases a radioactive scan is necessary to identify this tissue.
I 131 is a radioisotope that can be used for treating feline hyperthyroidism. It accumulates in the thyroid glands and subsequently destroys it. The amount of destruction is dependent on the amount of isotope given to the cat. It is an effective means of treating thyroid tumors. Radioactive iodine has the advantage of bypassing anesthesia, surgery, and anti-thyroid drugs and their associate risks. However, I 131 has a long half-life (8 days) necessitating prolonged hospitalizations (7-10 days), is potentially hazardous to humans, and has the potential to induce hypothyroidism due to its destruction of the thyroid gland. The treatment is done at a special veterinary center that is licensed to treat and handle the radioactive drug, and is available only on a referral basis by one of our doctors. Approximate cost is $850-$1000, but this may be more if the hyperthyrodism is severe.
In short, there are 3 different ways to treat hyperthyroidism. We find that in most cases surgery is the better way to go if the heart and body systems are in good working order. You create a cure of the disease very quickly. In most cases no medication is needed post-operatively.
Navigation Area
|
|
|
|
|
| 
| 
| 
| 
|
